One primary feature of the NGN is a separation between a service layer and a transport layer. The transport layer is based on packet and optical technologies and the service layer provides various multimedia services. An NGN functional architecture includes the service layer with a core of an IP Multimedia Subsystem (IMS) and the Internet Protocol (IP) based transport layer.
The service layer of an NON includes various IP application servers and service control subsystems, such as an IP multimedia subsystem, a Public Switch Telephone Network (PSTN)/Integrated Service Digital Network (ISDN) Emulation subsystem. Under the control of a Network Attachment Sub-system (NASS) and a Resource and Admission Control Sub-system (RACS), the transport layer of the NON provides IP connectivity between NGN terminals and hides the transport technology underlying an IP layer of access and core networks, implementing the separation and interaction between the service layer and the transport layer.
Various gateway functional entities, including Border Gateway Function (BGF), Media Gateway Function (MGF) and Signaling Gateway Function (SGF) of the transport layer, have abilities to interact with the service layer.
The BGF provides interfaces between two IP transport domains, and may be located at the border of a home network of a user, an access network or a core network. The BGF may provide the following functions:
gates opening/closing (the so called gate is to filter a message according to the IP address/port);
packet marking of outgoing stream;
resource allocation and bandwidth reservation of upstream/downstream;
allocation and conversion of IP addresses and ports;
remote Network Address Translation (NAT) traversal;
policing the incoming stream;
IP address anti-spoofing;
usage metering;
interworking between Internet Protocol version 4 (IPv4) networks and Internet Protocol version 6 (IPv6) networks; and
hiding topology.
One or more of the above function can be controlled through interactions between the BGF and a Resource and Admission Control Function (RACF) of the RACS.
The Usage Metering function refers to statistic reporting of accurate statistic information of a media flow. The media flow metering is mainly used for operation, maintenance and charging in the NGN.
However, the current drafts of the NGN standards does not specify metering information or metering method for media flows, that is, specific information of the media flow metering, issues on how to perform a media flow metering, how to control the metering and how to acquire and use the metering information remain to be addressed.